A power system may include distributed power sources, such as photovoltaic DC power or fuel cell DC power, to provide power supply to distributed power utilities. A power inverter is usually used between the power source and the loads to adapt the power supply to the power usage. For example, a power system may include a 3-phase inverter module that inverts the DC power to 3-phase AC power. The 3-phase AC power may be supplied to the loads.
Two types of loads are usually driven by the power supply in parallel: internal critical loads that are associated with the local loads at the distributed power source, and grid loads that are distributed among various utilities. The grid loads may be connected to the power inverter via an output contactor. When the grid loads are connected, the power inverter is said to work at a “grid-tie” mode, and when the grid loads are disconnected, the power inverter is said to work at a “stand-alone” mode. For example, when there is a shorting problem on the power transmission line, the grid loads will be disconnected, and the only loads supplied by the power will be the internal critical loads.
During the “grid-tie” mode and the “stand-alone” mode, the operation of the power inverter may be substantially different. Therefore, when the grid loads are suddenly disconnected, for example, due to a transmission line shorting, control of the power inverter may jump from one operation mode to the other operation mode. Such a non-smooth transition from a “grid-tie” mode to a “stand-alone” mode is typically known as an “islanding” condition. Conventionally, in order to maintain the operation of power inverter, two separate power controllers have to be configured separately for the two modes. For example, one controller may operate under “grid-tie” mode when the grid loads are connected, and the other controller may take over and operate under stand-alone mode when the grid loads are disconnected. However, the switch between the two power controllers may cause various issues. For example, it may cause a voltage dip that may shut down a sensitive local load such as a variable frequency drive (VFD).
One power converter system designed to prevent an overcurrent during the transition between the two operation modes is described in U.S. Pat. No. 6,304,468 to Ichinose et al. (“the '468 patent”). The power converter system described in the '468 patent includes a circuit breaker connecting a load to the power system. When the circuit breaker is closed, the power converter may change from a self commutated operation to a grid connected operation. The '468 patent describes a power converter controller that matches the phase of an output voltage of the power converter to the phase of the system voltage. In particular, the power converter controller uses the voltage on the system side of the circuit breaker to adjust the output phase of the converter system. According to the '468 patent, the circuit breaker can be closed when the output phase coincides with the phase of the system voltage.
Although the power converter system described in the '468 patent may be effective to make the transition between the two operation modes, it may nevertheless be suboptimal. For example, the power converter system described in the '468 patent may not be robust because it does not have effective islanding detection and prevention functions. Therefore, when an islanding condition occurs, a voltage dip may occur during the transition from the grid connected operation to the self commutated operation. As a result, local loads such as VFDs may be shut down due to the voltage dip, and the power system may be rendered inoperative.
In addition, an AC voltage may be characterized by three parameters, amplitude, frequency, and phase. Although the power converter controller of the '468 patent matches the phase of the converter output voltage to the system voltage, it does not effectively match the amplitude and frequency of these two voltages. For this additional reason, power converter system described in the '468 patent may not achieve a smooth grid-tie transition.
The disclosed power inverter control system and method are directed towards overcoming one or more of the problems set forth above.